Some varieties of geranium are very easy to split some can be bit stubborn. They don't need to be kept warm,over winter. If mine, I would pot on now, keep now in greenhouse but put outside when weather warms up a bit. Best in the ground though as soon as you can after that. When you pot on have a look at your plant....if its big enough can you pull it apart, tease it apart? Maybe just a couple of larger splits and pot these up